Viruses comprise a shell or capsid constructed of many copies of one or more relatively small proteins that encase genomic material. In this way viruses show maximum parsimony with their resource (the genome) while taking advantage of their host's resources to manufacture more viruses. A substantial number of viruses package their genomes as they assemble. These include alphaviruses, flaviviruses, retroviruses, hepadnaviruses, some bacteriophages and numerous plant viruses. In some cases the genome may serve as a scaffold that concentrates and organizes protein subunits while in others, it may cross link subunits and induce conformational change. Additionally, the genome may neutralize charges present in nucleic acid-binding domains to overcome electrostatic repulsion between protein subunits.
Assembly reactions must be carefully regulated to proceed successfully. In order for a virus to form, the capsid must assemble on the right nucleic acid and accessory proteins. If the capsid assembles with the wrong geometry, the defects in the resulting virus are likely to render it un-infectious. Similarly, binding the wrong nucleic acid or assembling without nucleic acid will lead to uninfectious virus-like particles. Thus, successful assembly must be triggered in response to a specific signal. Furthermore, in many cases assembly reactions have the ability to “proofread” defects to remove incorrectly positioned proteins. Finally, at the appropriate time capsids must release the genomic material; thus, virus lability is also important to virus lifecycle.
Therefore, virus assembly is a suitable target for antiviral agents, or the so-called Capsid Protein Allosteric Modulators (CpAMs). Drugs that interfere with viral capsid assembly hold potential as CPAM but have proven difficult to identify by conventional screening methods. An easy read out of virus assembly pattern in vitro will bring the benefits of large scale CPAM molecule screening. This disclosure provides a generalized assay system for virus capsid assembly and its use in small molecule assembly effector screening.